A progress in an electronic display as man machine interface has resulted in popularization of an interactive input system. Among others, an apparatus having a touch screen (a digitizer) and an associated display is widely used in various fields such as an ATM (automated teller machine), a merchandise management, an outworking (canvassing, selling), a guide sign, and an entertainment device. Use of the touch screen in combination with a lightweight and thin display (e.g., a liquid crystal display) dispenses with any keyboard and exhibits the features of the display, and thus such a touch screen display is increasingly used for mobile devices. The touch screen display is a device for inputting (or feeding) information or instructions to a computer by touching a prescribed area on a touch screen (or a touch sensitive panel) with an input means such as a finger or a pen (a digital pen or a stylus). According to the position detection method, the touch screen display can be classified into an optical system, an ultrasonic-wave system, a capacitive system, a resistive system, and other systems. Among these systems, the capacitive system detects the location of the touch using a change in capacitance. In light of excellent functionality, a projected capacitive touch screen display of ITO grid system is now being used for mobile devices and is receiving a lot of attention. The mobile devices include a smartphone, a mobile phone, an electronic paper, a tablet personal computer (PC), a pen tablet (or a graphics tablet), and a game console. Among the capacitive touch screen displays, a pen-input touch screen display which uses a pen as an input means, is also now being widely used and being increasingly applied for a smartphone, an electronic paper, a tablet PC, a pen tablet, a game console, and a PC. The display of the touch screen display is provided with, on a surface thereof, a hardcoat film, an anti-Newton-ring film, a soft film, or other films according to purposes. Nowadays, a pen-input tablet computer (tablet PC) as a pointing device is also in widespread use. The tablet PC and the above-mentioned touch screen display are referred to as a pen-input device. Moreover, the following are also developed: a high-resolution (4K) touch screen television having a pixel dimension (or pixel number) four times as large as that of a full high-definitions television, and a high-resolution pen-input device that is used in the architectural field or the medical field. For these devices, a high transparency is also required.
As pen-input devices are widely used for various purposes, the devices require higher functions for enabling smooth or excellent writing in pen input, e.g., for enabling smooth or excellent writing similar to writing on paper with a ballpoint pen. Unfortunately, a pen-input device provided with the soft film, which has an excessively strong resistance, never enables smooth writing similar to writing on paper with a ballpoint pen.
As a film having an improved writing smoothness, Japanese Patent Application Laid-Open Publication No. 6-309990 (JP-6-309990A, Patent Document 1) discloses a pen-input touch screen that comprises a transparent panel board having a surface pencil hardness of not less than 2H and a coefficient of dynamic friction to an input pen of 0.02 to 0.30. The panel board has an elastic deformation by which the panel board falls into a depth of 20 to 100 μm when contacted with an input pen under a load of 300 g and returns to the original state when the load is removed. According to this document, formation of a transparent adhesive layer having an elastic deformation underlying a surface-treated layer composed of a polysiloxane-series thermosetting hardcoat material improves abrasion resistance to an input pen and smooth writing with the input pen.
For the touch screen, although the transparent adhesive layer gives feeling of dent on pen input, the surface of the touch screen is too smooth and slippery. Thus, the touch screen fails to improve writing smoothness sufficiently.
Meanwhile, there has also been reported a film being smooth to writing touch (smooth in writing) improved by forming an uneven structure on a surface layer (such as a hardcoat layer) of the film. Japanese Patent Application Laid-Open Publication No. 7-244552 (JP-7-244552A, Patent Document 2) discloses a surface material of a pen tablet for a computer. The surface material has a transparent plastic film substrate and an ionization-radiation-curing-type resin layer formed on a surface of the substrate, and the ionization-radiation-curing-type resin layer has a surface having a 10-point average roughness (Rz) of 0.5 to 5.0 μm and an average distance (Sm) between peaks and valleys of 50 to 500 μm.
This surface material has an uneven structure formed by fine particles, and a high-definition display provided with the surface material unfortunately produces sparkling and has a low visibility.
A method for improving smooth writing by controlling a coefficient of friction is also known. Japanese Patent Application Laid-Open Publication No. 2003-296008 (JP-2003-296008A, Patent Document 3) discloses a surface material for a pen-input apparatus, and the surface material consists of a substrate and a surface layer and has a coefficient of dynamic friction of 0.02 to 0.5 and a coefficient of static friction of 0.4 to 1.5 when an input pen is moved on the surface of the surface layer at a load of 200 g and a rate of 10 cm/sec under an atmosphere of 20° C. and 50% relative humidity. This document discloses that the surface layer has a center line average roughness of 0.1 to 5 μm.
The surface material, which offers a large scratchy feeling (scratch in writing) at the beginning of pen-input operation probably due to a high coefficient of static friction of the surface layer, fails to give an excellent writing smoothness.
Japanese Patent Application Laid-Open Publication No. 2013-77135 (JP-2013-77135A, Patent Document 4) discloses a sheet member to be laminated on a touch surface of a touch screen. The sheet member has a surface having a difference (μs−μk) between a coefficient of static friction (μs) and a coefficient of dynamic friction (μk) of not more than 0.05. In working examples of this document, a sheet member having a coefficient of static friction of 0.16 to 0.24 and a coefficient of dynamic friction of 0.16 to 0.24 is prepared. The document also discloses that the sheet member is preferably made of a self-restoring resin. Working examples of the document describes, as a curable composition, a combination of 84 parts by weight of urethane acrylate and 15 parts by weight of a (meth)acrylate-modified polydimethylsiloxane, a combination of 78 parts by weight of urethane acrylate, 12 parts by weight of an active-energy-ray-curable polydimethylsiloxane-series copolymer, and 9 parts by weight of acryloyl morpholine, or other combinations.
Unfortunately, this sheet member has an insufficient writing smoothness. In particular, the sheet member fails to provide smooth writing similar to writing on paper with a ballpoint pen.